1. Field of the Invention
This invention pertains to auto focus systems for cameras and more particularly an auto focus system for a video camera utilizing electronic signal processing of the video signal to obtain focus.
2. Description of the Prior Art
Auto focusing systems for achieving the automatic focusing of a scene viewed through a still or moving camera are known in the prior art. Ideally, such systems accomplish the automatic focusing of the camera on a scene on which the camera is aimed without requiring any intervention on the part of the operator.
Known auto focus systems can be divided into two types: (1) active systems and (2) passive systems.
Active systems generally comprise the introduction of a signal emitted by the camera, which is reflected off the subject being viewed and detected by a sensor on the camera. In video cameras, the most popular active systems use infrared light as the signal. The system determines the distance that the light beam has travelled through and drives a focusing motor on the lens accordingly.
One disadvantage of the infrared auto focus system is that it can be fooled if a subject is viewed through a plane of glass. In such a case, the infrared beam is reflected off the pane of glass causing the camera to focus on the glass rather than on the subject behind it. This is a particularly important disadvantage when used with CCTV cameras in surveillance systems, since such systems often utilize cameras focusing through glass windows or doors.
A further disadvantage of active systems is that they do not work well over long distances due to the attenuation of the sensor signal.
Other active systems are sonar signals (ultrasonic) which bounce off the subject and are returned and detected by an ultrasonic sensor in the camera to adjust a focusing motor in a similar manner to the infrared system.
All active systems require the addition of at least a signal source and a signal detector. Additionally, signal processing circuitry will be required. Thus, bulk and expense will be added to the camera to accommodate the additional elements required.
In passive systems, the scene being viewed provides the necessary signal. In one passive system widely utilized in video cameras, a split image range finder similar to that used in 35 mm. still cameras is used and coupled to a charge coupled device (CCD). The charge coupled device is used to compare two images derived from the split image range finder. The resulting signal derived from the CCD is processed to calculate the camera to subject distance and in turn drive the focus motor. Alternative CCD bases systems utilize a system of prisms in which a separate split-image range finder is not required. The image is received instead through the lens (TTL) and bounced off a stationary mirror to a prism and then to a second mirror. The second mirror is then moved along with the lens, until the image appears in it.
The passive systems also have the disadvantage of requiring an additional sensor (in this instance the charge coupled device) which once again adds significantly to the cost and expense of the system. Additionally, the use of a separate split image range finder or the mirrors and prisms adds significantly to the bulk and weight of the camera as well as to its expense.
It is thus an object of the present invention to provide an auto focus system which does not require additional sensors or light sources.
A further object is to provide an auto focus system which will not be fooled when focusing through a pane of glass.
Still another object is to provide an auto focus system that is effective over long distances.
It is yet another object to provide an auto focus system which does not require additional and bulky devices in order to achieve auto focus.
These and other advantages of the present invention will become more apparent upon reference to the following specification and attached drawings.